In the technical field for making glass containers, it is known that there exists a risk that the containers have one or several localized areas with poor distribution of the material affecting the esthetics or more seriously the mechanical strength of the containers. It is known that small thickness or “thin” defects mainly form in specific regions of the container which have different radii of curvature such as the shoulder or the chime of the container.
In order to measure the thickness of the wall of a bottle, a so-called triangulation method is known, consisting of projecting a light beam onto the wall of the article with a non-zero angle of incidence, and of collecting the light beams reflected by the outer surface and the inner surface of the wall. These light reflections on both of these surfaces occur according to the laws of specular reflection of the incident beams, i.e. symmetrically to the incident beam relatively to the normal to the surface at the impact point of the incident beam.
Measurement of the thickness of the container 2 is for example known from patent EP 0 320 139 and as illustrated in FIG. 1, by sending a light beam B onto the wall of the container with an angle such that a portion C of the light beam is reflected by the outer surface of the wall and that a portion of the beam is refracted in the wall and then reflected D by the inner surface of the wall. The rays C, D reflected by the inner and outer surfaces of the wall are collected by a lens E in order to be sent on a linear light sensor F. The plane containing the optical axis, the linear sensor and the median radius of the incident beam is currently called the triangulation plane. The thickness of the wall of the container is measured depending on the separation, at the light sensor, between the beams reflected by the inner and outer surfaces of the wall. The container is driven into rotation so as to perform one revolution in order to measure its thickness along one of the transverse cross-sections. Advantageously, the inspection cross-section is located in an area of the container having a great risk of forming thin areas such as the chime or the shoulder.
An alternative to the previous technique consists of using an array sensor instead of a linear sensor in order to measure the glass thickness and therefore its distribution around the whole of the height of the resulting field covered by the array sensor provided with an objective. In this configuration, the light beam produced by the source extends perpendicularly to the triangulation plane so as to vertically cover the field of the array sensor.
Because of the geometrical shape of the container to be monitored and/or because of the lack of parallelism between the inner and outer surfaces of the wall to be measured, the deviations of the directions of both specular reflections may vary by several degrees. Thus, as illustrated in FIG. 1, the geometrical shape of the article may cause significant variation of the position of the impact point of the incident beam so that the reflected rays C′, D′, may have strong deviations relatively to the optical axis and the points from which they stem may have a large position deviation. Also, as illustrated in FIG. 1A, the lack of parallelism between the inner and outer surfaces of the wall to be measured may lead to reflected rays C″, D″ having strong deviations relatively to the optical axis.
A method for measuring the thickness of an object is also known from patent FR 2 069 220, consisting of projecting a narrow light beam onto the object so that the light beam successively produces a spot of light on the external face and the internal face of the object. A lens is positioned relatively to the object in order to form real images of the light directly reflected or diffusively reflected by the inner and outer surfaces, like two concentrated light points spaced apart on a screen. The distance between both of these spots is measured with any device such as for example, a vidicon or an image analyzer.
According to a preferred alternative embodiment, the lens is positioned so as not to receive the light rays which are directly reflected by the inner and outer surfaces according to angles equal to the angles of incidence on these surfaces. This technique, which intends to form real images of the diffusively reflective light, cannot be notably applied for monitoring the thickness of the walls of bottles since the light is not reflected on the walls in a diffusive way.
However this patent provides the case when the lens is provided for collecting a directly reflected ray. This patent however specifies that in such a case, a small change in the surface angle of the object changes the angle of the reflected ray, requiring significant motion of the lens for collecting this reflected ray.
The technique described in this patent is not industrially applicable as it is inconceivable to move the lens for collecting the reflected rays.
Therefore appears the need for being able to have a facility for measuring the thickness of the wall of transparent or translucent containers operating for a wide range of containers and/or under significant deviation conditions of the beams reflected by the wall and/or under significant position deviation conditions of both reflection points.
However, in the field of the design of focusing optics such as objectives, it is known that extreme conditions, notably collecting rays forming a large angle with the optical axis and/or stemming from points of the object plane away from the optical axis, or else large incidences on the image sensors, lead to optical aberrations and light losses, which are detrimental to the operation of the sensor and of the objective or else are costly and complex to correct.
The present invention aims at finding a remedy to the drawbacks of the prior art by proposing a performing and economical facility for measuring the thickness of the wall of transparent or translucent containers, operating for a wide range of containers and/or under significant deviation conditions of the beams reflected by the war and/or significant position deviation conditions of both reflection points.